1. Field of the Invention
The present invention relates to a light guide plate for surface light source device and a method of manufacturing it and, more particularly, a light guide plate for a surface light source device used as a backlight for a liquid crystal display for OA equipment, a television set, a measuring instrument, a watch, or the like, and a method of the light guide plate by injection molding.
2. Description of Related Art
A surface light source device is well known. A small surface light source device is used as a backlight for a liquid crystal display wristwatch, and a large surface light source device is used as a backlight for an advertising display panel or an illumination device for a show window. An LED is used as the light source for the small device, and a fluorescent tube is used as the light source for the large device. As a light guide plate, an acrylic plate cut so as to have a proper dimension is used.
Since a surface light source device is required to have a small thickness and to cause a predetermined plane to emit uniform light, a light source is generally arranged at a lateral position of the light guide plate. For this reason, the light guide plate has been subject to various processings which include shaping to a special shape, surface roughening with sandpaper, special tool or apparatus and polishing of incident surface to making it specular.
In recent years, with epochmaking progress of a liquid crystal display technique and development of OA equipment, electronic-communication equipment, or the like, demand for a surface light source device used in a liquid crystal display device having a size of about 10 inches steeply increases. As a light source arranged at a lateral position of a light guide plate, a long-life extremely slender fluorescent tube having a diameter of 4 mm or less is developed. Light guide plates have come to be manufactured by injection molding which has a small number of steps and provides mass-production of light guide plates with stable quality.
A light guide plate used in a surface light source device is required to be variously improved to make it possible that a plane having a designed area emits source light guided in a latitudinal direction as uniform plane light. Various proposals associated with the improvements have been made. Almost all the various proposals use one of the following techniques: a technique in which a surface opposing the emission surface, i.e., a reflecting surface, is processed (formation of an uneven surface, coating, or printing) by some means to modify its reflectance distribution; a technique in which a reflecting surface is arranged not parallel to an emission surface, but formed by various planes and curves; and a technique obtained by combining both the above techniques to each other.
In these techniques, a light guide plate having a reflecting surface not parallel to the emission surface is thick at the incident surface for source light or near the incident surface and thin at positions distant from the incident surface. Such light guide plates are disclosed in Japanese Patent Laid-open No. 3-59526, Japanese Utility Model Laid-open No. 3-104906, Japanese Utility Model Laid-open No. 5-75738, or Japanese Utility Model Laid-open No. 5-75739, and in FIG. 7, FIG. 8. FIGS. 7 and 8 are side views showing light guide plates. Referring to FIGS. 7 and 8, each upper surface is the incident surface, and each left surface is the emission surface. The present invention relates to a light guide plate having a shape as shown in FIG. 7 or 8 and to an improvement of a method of manufacturing the light guide plate.
A conventional method of molding a light guide plate is described here with reference to a light guide plate having a typical shape shown in FIGS. 9 and 10. FIG. 9 is a plan view showing the light guide plate when viewed from the emission surface side, and FIG. 10 is a right-side view of FIG. 9. Therefore, referring to FIGS. 9 and 10, an upper surface is the incident surface 1 which is perpendicular to the emission surface 2 in FIG. 10. Since a fluorescent tube is arranged in the longitudinal direction of the incident surface at a position outside and near the incident surface, the thickness of the light guide plate is defined in consideration of the diameter of the fluorescent tube.
A reflecting surface 3 is obliquely formed with respect to the emission surface 2 so that the reflecting surface 3 can directly reflect incident light from the incident surface 1. Thus, the thickness of the guide plate gradually decreases downward in FIGS. 9 and 10.
When the light guide plate is to be formed by injection molding, a specific position on a specific surface for a gate (opening through which molten material is injected into a mold cavity) of the mold must be determined. Since the light guide plate is used in a surface light source device, the gate position is determined in consideration of the utilization efficiency of light, uniform emission from a large emission surface, profitability and so on. Judging from this point of view, it is conventionally assumed that the incident surface should be specular to efficiently receive source light. In addition, all the surface should be specular to obtain uniform outgoing light.
According to conventional understanding, a gate arranged on the incident surface provides an increase in cost because the incident surface must be made specular by high-accuracy polishing or buff finish serving as post-processing. Therefore, such arrangement of gate on the incident surface has not been employed.
The emission and reflecting surfaces generally must have effective areas as large as possible. In addition, in order to obtain uniform outgoing light, an uneven surface having a variety of shapes such as a net-like pattern is often formed on the reflecting surface, otherwise coating or printing is often applied to the reflecting surface. Therefore, it has been avoided to arrange a gate on these surfaces.
Referring to FIGS. 9 and 10, a gate may be arranged on the lower surface 4. However, the light guide plate has a very small thickness at lower surface 4. For example, an ultra-thin light guide plate for office Automation (“OA”) equipment having a size of about 10 inches used, the thickness at the lower surface is about 1 to 2 mm, and a gate can hardly be arranged on the lower surface. Even if a gate is arranged on the lower surface, a sufficient injection pressure cannot be obtained by a general molding machine, an acrylic molten material cannot be preferably injected into the cavity. Thus, transferring characteristics are considerably degraded.
In order to increase the injection pressure, or increase the temperature of the mold to make the flow of the material easy, a highly expensive molding machine of high-accuracy control is required. For this reason, the cost of the light guide plate inevitably increases. Furthermore, the number of gates may be increased. In this case, however, the cost of the mold increases and a so-called weld line is inevitably formed, leading to a fatal problem for uniform outgoing light. Therefore, it is not practical to arrange a gate on the lower surface.
For these reasons, a gate is conventionally arranged at a position on one of side surfaces 5 and 6 in FIG. 9, in particular, in the thick portion near the incident surface. Similarly, the above arrangement is employed in molding for a two-light light guide plate in which the inclination direction of the reflecting surface 3 with respect to the emission surface 2 is reversed at the central portion of the reflecting surface 3, the lower surface 4 has a thickness almost equal to that of the incident surface 1 so that another fluorescent tube is arranged outside and near the lower surface 4, as shown in FIG. 11.
However, according to a conventional molding method, as shown in FIG. 9, if a gate G is arranged on the side surface 5, the material quickly flows in a flow path A, but slowly flows in a flow path B at injection. Thus, the material does not uniformly flow from the incident surface 1 to the lower surface 4 and the flow varies depending on the position. In addition, the flow on the side surface 5 is considerably different from the flow on the side surface 6. For this reason, the pressure difference and temperature difference between regions fall into disorder.
As a result, in particular, when an uneven surface having a variety of shapes is formed on the reflecting surface, the shape of the uneven surface cannot be desirably transferred. In addition, problems such as formation of an weld line and warp after molding arise easily. In the ultra-thin light guide plate as shown in FIGS. 9 and 10, since a material cannot completely filled in a cavity by molding machine and an ordinary molding method, a troublesome examination must be performed for conditions set for molding processing at every slight change in shape of the light guide plate.